Distortion reduction on modulated amplifiers



Jan. 29, 1946. RQMANDER I I 2,393,709

DISTORTION REDUCTION ON MODULATED AMPLIFIERS Filed NOV. 16, 1942 INVENTOR. flue-o IPOMAA/DEI? atented Jan. 29,. 1946 DISTORTION REDUCTION ON MODULATED AMPLIFIERS Hugo Romander, North Caldwell, N. J., assignor to Federal Telephone & Radio Corporation, New York, N. Y., a corporation of Delaware Application November 16, 1942, Serial No. 465,712

7 Claims.

This invention relates to the prevention of distortion and in particular to the use of impedance inverting circuits as a means for preventing distoltion in modulated class C amplifiers.

The operation of a modulated class C amplifier is well known in the art and therefore will not be described in detail. What may be considered its chief characteristic is the high negative bias which is applied to the amplifier grid. This bias may have a value far greater than that required to reduce the plate current to zero or cut-off in the absence of any excitation voltage. When radio frequency excitation voltage is applied to the amplifier input a considerable portion of the voltage on the positive half of the cycle is required to overcome the cut-off bias with the result that the plate current fiows only for a portion of the positive half of the radio frequency cycle. As is well understood in the art these conditions resuit in greater efficiencies than those in which the plate current is permitted to flow for a consider= abl portion of the cycle.

Modulation is effected by superimposing a modulating voltage upon the normal plate supply voltage. For 100% modulation the peak value of the modulating voltage is made equal to the nor= mal plate supply voltage. If the amplifier characteristic were linear, the plate current at the peaks of the modulation cycle, at which the plate voltage is doubled, would be double that during periods of no modulations. Such linearity would result in no distortion and the amplifier output would be a true amplified reproduction of the am plifier input. Unfortunately, these ideal conditions do not occur in practice and there is a de-= cided variation of the amplifier output from the ideal conditions at both the peaks and valleys.

of the higher modulating voltages. This is the distortion which it is the purpose of my invention to reduce or correct.

It is known that as the plate voltage of an amplifier increases, the input impedance in creases. This may be explained by the fact that the increased plate voltages rob the grid of electrons which would be normally attracted thereto at lower plate voltages. In accordance with my invention I make use of this change of impedance to vary the driving voltage to the amplifier in put. This is accomplished by connecting an impedance inverting network between the output of the driver stage and the input of the amplifier stage as will be explained in detail hereinafter.

The prior art discloses the use of impedance inverting networks in linear power amplifiers wherein a modulated carrier wave is applied to the input of the amplifier and a constant D.-C. potential is applied to the anode of the amplifier tube. In this type of amplifier the input impedance decreases on the positive peaks of the modulation cycle, caused, as is Well known, by the grid current which flows as the grid swings positively. One well known method of correcting, for distortion in amplifiers of this type is to employ two tubes in parallel branched circuits, one tube operating continuously and the other tube operating only on the positive modulation peaks at which the distortion occurs.

It is an object of my invention to reduce distortion in a modulated class C amplifier.

Another object of my invention is to vary the driving voltage of a modulated class C amplifier during periods of modulation in which the amplifier output is not a true reproduction of the amplifier input.

Another object of my invention is to increase the operating efiiciency of a modulated class C amplifier.

Other objects and advantages of my invention will appear as I proceed with the following detailed description taken in connection with the accompanying drawing in which;

Fig. i is a circuit diagram illustrating one embodiment of my invention wherein a quarterwavelength line is employed as an impedance in verting element; a

Fig. 2 is a circuit diagram illustrating a second embodiment of my invention wherein a balanced T-section is employed as an impedance inverting element, and;

Fig. 3 is a circuit diagram illustrating a third embodiment wherein a balanced 1r-S8Ctio-11 is employed as an impedance inverting means.

Referring to Fig. 1, D represents a carrier wave amplifier employed to excite or drive the input of a modulated class C amplifier, A, through an impedance inverting network, N. The driver stage consists of two tubes, l and I, connected in a push-pull circuit. The input circuit of the driver consists of an inductor 2 and a capacitor 3 tuned to the operating carrier frequency. The extremities of the input circuit are connected to the grids 4 and 4' of the tubes i and I respectively. The center point of the inductor 2 is connected to grounded cathodes 5 and 5 through a biasing means shown as a capacitor and grid leak combination 6. The inductor 2 acts as the secondary of a transformer of which the primary is the inductor l and which may be connected to the output of a previous amplifier stage, not shown.

The anodes d and 8' are connected to the output circuit of the driver through blocking capacitors 9 and. 9 and to a source of positive potential through choke coils l and Ill. The output circuit i I consists of a balanced 1r-network composed of capacitors l2 and L3 and inductors l4 and I4,

tuned to the frequency of the carrier wave and with constants so chosen that the circuit functions as an impedance changing network. The output circuit is coupled to an impedance inverting network, N, which is illustrated as a quarter wavelength line. The output of the quarter wave line is connected to the input circuit l5 of the '20 and 20' are connected across the output of circuit I5. Biasing voltage for the grids 20 and 20' is obtained in any suitablemanner such as for example by means of the condenser and grid leak combinations 2I-22 and 2l'--22'. I also prefer to employ a fixed bias in addition to the self bias. The fixed. bias. may be obtained from a battery 23 with its negative terminal connected to the junction point of leak resistors 22 and 22, and with its positive terminal connected to the tube cathodes. The anodes, '24 and 24', are connected to the opposite ends of an output circuit 25 consisting of an inductor 26 and a capacitor 21. A load circuit, for example, an antenna not shown, may be coupled to the output circuit 25 through blocking condensers 28 and 28'.

The center point of the inductor 26, which is substantially at a radio frequency ground potential, is connected to the positive terminal of a power source through the secondary winding 29 of a modulation transformer 30. The power for modulating the class C amplifier is obtained from the final stageiof a voice or signal amplifier schematically shown at B. This final stage is usually operated class B, but the circuits therefor are not shown in detail since they form no part of the present invention. The voice amplifier as a whole is designed to develop a maxi- I mum undistorted modulating potential across the secondary winding 29 equal in value to the potential of the direct current power source supplying the amplifier A. This is desirable in order to obtain 100% modulation of the carrier wave.

The manner in which the circuits are adjusted in order to prevent or substantially reduce distortion in accordance with my invention will now bedescribed. It is assumed that for the required power output, suitable amplifier tubes,

' power supply voltages, biasing voltages, etc.', have been chosen. This information may be obtained put impedance matches the surge impedanc of :the line N at carrier frequency while its input impedance assumes a value suitable for serving as an output impedance for the plate circuit of the driver amplifier D. The values forthese impedances are not critical but should be capable of developing the required voltage and power for driving the grids of the amplifier A. Amplifier D should not be loaded to its maximum capacity but should have good regulation and a reserve power capacity of approximately 50% of its normal output without modulation. In other words, during modulation periods when the driver will be called upon to deliver increased power for excitation, this power should be available.

As aforementioned, the line N is a quarter wave length long and functions as an impedance inverting network. In accordance. with well known would be Zo /Z, where Z0 is the surge impedance In accordance with my invention,

of the line. the impedance Z is the impedance looking into the network IS without modulation and is made from the tube ratings as supplied by tube manu- "latter is suitable for giving the required voltage swing to the grids of amplifier A. These adjustments are made at carrier frequency and with no modulation voltages applied to the anodes of the amplifier.

The network II is next adjusted until its outequal to Z0.. The impedance looking into the line N is therefore also Z0 and the line is flat, that is, contains no standing waves and there are no reflections from the output to the imflit end of the line.

After the circuits have been adjusted for satisfactory operation without modulation, a modulating voltage is applied as above described and as shownin Fig. 1. For moderate values of modulation voltages the envelope of.the modulated carrier wave is substantially a replica of the modulation voltage. This is true whether or not the impedance inverting network is inserted in the coupling between the driver and the amplifier stages. However, if the impedance inverting network were omitted there would be a decided distortion of the output as modulation is approached. But with the impedance inverting network, the following operation takes place.

At the higher modulation levels the input impedance of the modulated amplifier increases on the positive peaks and decreases on the valleys or negative peaks. The line is now no longer fiat and the increased impedance is reflected to the input end of the quarter wave line as a reduction of impedance. This reduction in impedance is passed on to the output of the driver stage through the impedance transforming net-' work II with the result that the radio frequency output of the driver stage increases. This increased output produces a greater radio frequency voltage swing on the grids-of the modulated amplifier with the result that the amplifier output also increases and tends to compensate for what would otherwise be a distortion in i the modulated carrier wave.

On the negative peaks of the modulation cycle the reduced input impedance of the modulated amplifier is reflected into the input end of the quarter wave line as an increased impedance with the result that the driver stage suffers a reduction in output, and the radio frequency driveto the grids of the amplifier is likewise reduced.

It will be seen that as long as the amplifier input impedance remains unchanged the radio frequency carrier excitation is maintained con= stant, while any change in impedance causes the driver to automatically increase or decrease the amplifier excitation in accordance with the amount and trend of the distortion.

. The above description and explanation of the operation of my invention has been given by referring to the impedance inverting network as a quarter wave length line. This network may have either distributed or lumped constants, the only limitation being that it functions electrically as a means for inverting any impedance or impedance variation which may occur on its output side and for reflecting this inverted impedance into the output circuit of the driver amplifier.

Fig. 2 shows a network N in which the impedance inverting means consists of a balanced T-section in which the constants of the inductors 30, 30', 3| and 3! and the capacitor 32 are so chosen that the network has the properties of a quarter wave length line. The figure also illustrates a different type of impedance transforming network from that shown in Fig. 1 as networks ii and I5. In Fig. 2, the radio frequency transformers Ill] and I50 serve the same purpose as do the networks i I and I5 of Fig. 1.

Fig. 3 shows a network N" which takes the form of a balanced 1r-section, in which the electrical constants of the inductors 33 and 33 and the capacitors 34 and 35 are so chosen that the section functions as a quarter wave length line. In this case it will be obvious that the separate capacitors i3 and 34 may be combined into one unit, and that the same is also true of the capacitors 35 and IS. The impedance inverting network N" and the impedance transforming networks II and IE will then lose their separate identities but the operation of the circuits will be the same as before.

The circuits of all figures have been shown as push-pull circuits, but the invention is equally applicable to single-sided circuits. Many other modifications other than those specifically shown will occur to those skilled in the art but it is intended that my invention be not limited excent in accordance with the following claims.

What is claimed is:

1. In combination, a modulated amplifier of the type wherein the modulating voltage is applied solely to the plate of the amplifier and the unmodulated excitation voltage is applied solely to the grid, a driver amplifier for Supplying said unmodulated excitation voltage, and an impedance inverting circuit between said amplifiers, whereb said impedance inverting circuit passes only unmodulated excitation voltage.-

2. A modulated amplifier of the type wherein the modulating potential is applied solely to the plate thereof, and wherein an unmodulated excitation voltage is applied solely to the grid thereof, a source of voltage for supplying said excitation, and a network having both impedance inverting and impedance transforming characteristics connected between said unmoduand output circuits, means for biasing the grid of said amplifier beyond plate current cut-oil in the absence or radio frequency grid excitation,

a direct current supply voltage, and a modulating voltage of substantially equal maximum magnitudes applied to the plate of said amplifier, a driver amplifier capable of supplying an excitation voltage of sufiicient magnitude to drive the grid of said amplifier positive during a portion of the excitation voltage cycle and an impedance inverting network connected between said driver amplifier and said first mentioned amplifier.

4. In combination, a modulated amplifier of the class C type and having an input circuit, means for modulating solely the anode thereof, a driver amplifier having an output circuit for supplying unmodulated excitation voltage solely to the grid of said modulated amplifier, and an impedance inverting circuit carrying only unmodulated excitation voltage and coupling the output circuit of said driver amplifier to the input circuit or said modulated amplifier, said input circuit having characteristics for matching the impedance of said inverting circuit to the input impedance of said modulated amplifier, and for automatically reflecting changes in said input impedance, via said inverting circuit, to the output circuit of said driver amplifier.

5. In combination,-a modulated amplifier of the class C type, means for modulating solely the anode thereof, a driver amplifier having an output circuit for supplying unmodulated excitation voltage solely to the grid of said modulated amplifier, a plurality of circuits for coupling the output circuit of said driver amplifier to the grid of said modulated amplifier, one of said plurality of circuits comprising an impedance inverting network and another of said circuits being an impedance transforming circuit for matching the impedance of said inverting network to the input impedance of said modulated amplifier, and for automatically reflecting changes in said input impedance, via said inverting circuit, to the output circuit of said driver amplifier.

6. In combination a modulated amplifier of the type wherein the modulating voltage is applied solely to the plate of the amplifier, a driver amplifier having an output circuit for supplying an unmodulated excitation potential to the input of said modulated amplifier, a line having an odd multiple of quarter wave lengths at the excitation frequency connected between the output 01' said driver amplifier and the input of said modulated amplifier and means for matching the surge impedance oi. said line to the input impedance of said modulated amplifier, and for automatically reflecting changes in said input impedance, via said inverting circuit, to the output circuit of said driver amplifier. I

7. The method or using an impedance inverting network for correcting distortion in a modulated amplifier in which the modulating potential is applied directly to the anode output circuit of said amplifier which comprises deriving an unmodulated exciting potential from a source having good regulation and transmitting said potential through said impedance inverting network to the input circuit 01' said modulated amplifier, said modulation taking place solely in said anode circuit and said source supplying solely unmodulated exciting potential.

nooo aowmnm 

